Iron powder and the like generated with machining or internal abrasion are suspended as contaminants of fine ferromagnetic particles in oils or liquids such as machine lubricant or machining oil. The oils or liquids including the contaminants cause problems such as a decrease in machine drive reliability and a decrease in machinability and cleaning efficiency. Accordingly, a filter device has been proposed which can remove contaminants of fine ferromagnetic particles from the oils or liquids. For example, a magnetic-separation oil purifier described in PTL 1 includes a filter medium formed of magnetic alloy and a magnetizer applying a magnetic field to the filter medium, in which fine amorphous-alloy wire bundle is used as the magnetic filter medium and a permanent magnet is used as the magnetizer.
In an oil purifier described in PTL 2, a magnet producing a magnetic field and a liquid-transmitting inner tube are disposed in an outer shield tube of a rectangular tubular shape.